The electrical grid in many developed countries is a large-scale, distributed, cooperative system that functions to deliver power from production facilities such as hydroelectric generators, solar and wind farms, and fossil-fuel plants, across high-voltage transmission lines, to lower-voltage local distribution systems. Portions of the system include protective devices to prevent faults and failures in one area from affecting or damaging equipment in other areas.
When a short circuit occurs (e.g., due to power lines downed during a storm), the system voltage in the region drops to or near zero. Until the short is cleared, power transfer from generators to motors will be very low. If the current into a shorted region could be limited, the voltage drop could be similarly ameliorated. For example, if the system voltage only fell to 50% of nominal (rather than to only a few percent of nominal, or even zero), then power transfer from generators to motors could be maintained at near-normal levels, until the short circuit is cleared.
High-voltage series reactors having fixed impedance and no moving parts are occasionally placed in series with load conductors. These reactors have a fixed impedance on the order of one to a few ohms. The impedance limits system short-circuit current proportionally to the system voltage (by Ohm's Law, I=V/R).
Another known device type of relevance to the invention is a solenoid. These are wound coils with a moveable pole piece that are typically used in a shunt configuration (i.e., as the load across a voltage source). Excessive current may be avoided by providing a large number of windings, or by limiting the source voltage. A solenoid turns electrical power into mechanical work by moving the pole piece; the motion may ring a doorbell or latch or unlatch a car door, for example.